Method and apparatus for a differential feedback in an active impedence feedback circuit

ABSTRACT

A method and apparatus is provided for performing an active impedance differential current feedback. A signal is received. A current feedback of an output signal in an active feedback synthesis mode is performed based upon the signal. The active current feedback includes using an active feedback network for conditioning an output signal, converting the signal into a differential current signal, and summing at least two components of the differential current signal for performing a differential feedback.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to telecommunications, and, moreparticularly, to providing a differential current feedback in an activeimpedance feedback circuit for signal reception and/or transmission.

2. Description of the Related Art

In communications systems, particularly telephony such as a Plain OldTelephone System (POTS), it is common practice to transmit signalsbetween a subscriber station and a central switching office via atwo-wire, bi-directional communication channel. A line card generallyconnects the subscriber station to the central switching office. Thefunctions of the line card include supplying talk battery, performingwake-up sequences of circuits to allow communications to take place, andthe like. Voltage signals are processed and conditioned when beingdriven onto telecommunication lines.

POTS was designed primarily for voice communication, and thus providesan inadequate data transmission rate for many modern applications. Tomeet the demand for high-speed communication, designers have soughtinnovative and cost-effective solutions that would take advantage of theexisting network infrastructure. Several technological solutionsproposed in the telecommunications industry use the existing network oftelephone wires. A promising one of these technologies is the DigitalSubscriber Line (xDSL or DSL) technology.

xDSL is making the existing network of telephone lines more robust andversatile. Once considered virtually unusable for broadbandcommunications, an ordinary twisted pair equipped with DSL interfacescan transmit video, television, and very high-speed data. The fact thatmore than six hundred million telephone lines exist around the world isa compelling reason for these lines to be used as the primarytransmission conduits for at least several more decades. Because DSLutilizes telephone wiring already installed in virtually every home andbusiness in the world, it has been embraced by many as one of the morepromising and viable options.

There are now at least three popular versions of DSL technology, namelyAsymmetrical Digital Subscriber Line (ADSL), Very High-Speed DigitalSubscriber Line (VDSL), and Symmetric Digital Subscriber Line (SDSL).Although each technology is generally directed at different types ofusers, they all share certain characteristics. For example, all four DSLsystems utilize the existing, ubiquitous telephone wiringinfrastructure, deliver greater bandwidth, and operate by employingspecial digital signal processing. Because the aforementionedtechnologies are well known in the art, they will not be described indetail herein.

DSL and POTS technologies can co-exist in one line (e.g., also referredto as a “subscriber line”). Traditional analog voice band interfaces usethe same frequency band, 0–4 Kilohertz (KHz), as telephone service,thereby preventing concurrent voice and data use. A DSL interface, onthe other hand, operates at frequencies above the voice channels, from25 KHz to 1.1 Megahertz (MHz). Thus, a single DSL line is capable ofoffering simultaneous channels for voice and data. It should be notedthat the standards for certain derivatives of ADSL are still indefinition as of this writing, and therefore are subject to change. DSLsystems use digital signal processing (DSP) to increase throughput andsignal quality through common copper telephone wire. It provides adownstream data transfer rate from the DSL Point-of-Presence (POP) tothe subscriber location at speeds of up to 1.5 megabits per second(MBPS). The transfer rate of 1.5 MBPS, for instance, is fifty timesfaster than a conventional 28.8 kilobits per second (KBPS) transfer ratetypically found in conventional POTS systems.

DSL systems generally employ a signal detection system that monitors thetelephone line for communication requests. More specifically, the linecard in the central office polls the telephone line to detect anycommunication requests from a DSL data transceiver, such as a DSL modem,located at a subscriber station. There are multiple types of signalsthat are received and transmitted over multiple signal paths duringtelecommunication operation. Many times, feedback configurations in theamplifiers that process the transmission signals cause noise and powerproblems.

Many of today's amplifier circuits may call for electronic componentsthat have high bandwidth, which may increase consumption of power. Manytimes, power consumption in the line card can become undesirably high.Amplifier circuits that are used to condition communication signalsoften consume large amounts of power. Excessive power use can compromisethe effectiveness of line cards, particularly for remote line cards,which rely upon portable power supplies. Excessive power consumption canalso require additional resources to counteract the effects of highpower consumption, such as additional cooling systems to keep line cardcircuitry in operating condition. Excessive power consumption can alsorequire additional circuits to furnish the required amounts of powerneeded for efficient operation of line cards. Excessive powerconsumption can cause appreciable inefficiencies in the operation ofline cards and the communication system as a whole.

The prior art implementations of signal conditioning circuits generallyimplement signal feedback configurations that generally take the outputvoltage signal and then feed the voltage signal back to a negative inputof an amplifier within a circuit. In other words, the direct outputvoltage signal is the feedback signal used in the implementationsdescribed above. Among the problems associated with the currentimplementations, include the fact that a larger signal is fed back intothe circuit described above. The problem with such an implementation isthat larger signals may generally carry larger amounts of noise.Therefore, feeding back larger signals amounts to feeding back largeramounts of noise into the circuit, this may cause performance problemsin the amplifier circuit.

Additionally, feeding back the output voltage signal may requireamplifiers that have relatively large bandwidth capabilities. Utilizingamplifiers with larger bandwidth capabilities generally increases powerconsumption. Additionally, many prior art systems employ feedbackconfigurations that use single-ended feedback signals. The circuits thatuse these types of configurations may experience an excessive amount oflongitudinal signals, such as longitudinal currents. Longitudinalsignals may enter one of more amplification stages within a circuit andcause excessive noise. Utilizing the current methodologies, theperformance of a signal conditioning circuit may be compromised.

The present invention is directed to overcoming, or at least reducingthe effects of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method is provided forperforming an active impedance differential current feedback. A signalis received. A current feedback of an output signal in an activefeedback synthesis mode is performed based upon the signal. The activecurrent feedback includes using an active feedback network forconditioning an output signal, converting the signal into a differentialcurrent signal, and summing at least two components of the differentialcurrent signal for performing a differential feedback.

In another aspect of the present invention, an apparatus is provided forperforming an active impedance differential current feedback. Theapparatus of the present invention includes a first amplifier and asecond amplifier to buffer a differential input signal to generate adifferential output signal. The apparatus also includes an activeimpedance network that is adapted to condition the differential outputsignal and sum at a first component and a second component of a firstportion of the conditioned differential output signal. The activeimpedance network is also adapted to and to sum first and secondcomponents of a second portion of the conditioned differential outputsignal for generating a differential feedback signal for feedback intothe first and second amplifiers.

In another aspect of the present invention, an apparatus is provided forperforming an active impedance differential current feedback. Theapparatus of the present invention includes a voltage-to-current signalconverter to convert a differential input voltage signal and a firstbuffer amplifier operatively coupled to the voltage to current signalconverter. The first buffer amplifier is adapted to receive a componentof a converted differential input signal on a first input terminal andproduce a first buffered current signal. The apparatus also includes afirst sense resistor operatively coupled with the buffered amplifier anda second buffer amplifier operatively coupled to the voltage-to-currentsignal converter. The second buffer amplifier is adapted to receive acomponent of the converted differential input signal on a second inputterminal and produce a second buffered current signal. The apparatusalso includes a second sense resistor operatively coupled with thesecond buffer amplifier; an active feedback impedance networkoperatively coupled with the first and second sense resistors; and afirst datasense amplifier operatively coupled to the active feedbackimpedance network and to the first sense resistor. The first datasenseamplifier is adapted to generate a first and second component of adifferential feedback signal. The apparatus also includes a seconddatasense amplifier operatively coupled to the active feedback impedancenetwork and to the second sense resistor. The second datasense amplifieris adapted to generate a third and fourth component of the differentialfeedback signal. The apparatus also includes a first summing node forsumming the first and second components of the differential feedbacksignal to generate a first component of a summed differential feedbacksignal for feedback into the first buffer amplifier and a second summingnode for summing the third and fourth components of the differentialfeedback signal to generate a second component of the summeddifferential feedback signal for feedback into the second bufferamplifier.

In another aspect of the present invention, a system is provided forperforming an active impedance differential current feedback. The systemof the present invention comprises a subscriber line. The system alsoincludes a line card electronically coupled with the subscriber line.The line card is adapted to receiving a signal. The line card is alsocapable of performing a current feedback of an output signal in anactive feedback synthesis mode based upon the signal. Performing theactive current feedback includes using an active feedback network forconditioning an output signal, converting the signal into a differentialcurrent signal, and summing at least two components of the differentialcurrent signal for performing a differential feedback.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 illustrates a first embodiment of an apparatus in accordance withone illustrative embodiment of the present invention;

FIG. 2 illustrates an implementation of a line card into the apparatusdescribed in FIG. 1 in accordance with one illustrative embodiment ofthe present invention;

FIG. 3 illustrates a more detailed depiction of the line card inaccordance with one illustrative embodiment of the present invention;

FIG. 4 illustrates a simplified block diagram depiction of the SLICdescribed in FIG. 3, in accordance with one illustrative embodiment ofthe present invention;

FIG. 5 illustrates a block diagram depiction of a signal conditioningunit in the SLIC of FIG. 4, in accordance with one illustrativeembodiment of the present invention;

FIG. 6 illustrates a more detailed block diagram depiction of the SLICof FIG. 4, in accordance with one illustrative embodiment of the presentinvention;

FIG. 7 illustrates a circuit diagram of the signal amplifier in the SLICof FIG. 4, in accordance with one illustrative embodiment of the presentinvention; and

FIG. 9 illustrates a flowchart depiction of a method in accordance withone illustrative embodiment of the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

Embodiments of the present invention provide for a method and apparatusfor utilizing a current feedback configuration to reduce noise and/orpower consumption in an amplifier circuit used to condition electricalsignals (e.g., a communications signal). Embodiments of the presentinvention provide for feeding back a smaller current signal relative toan output signal in a signal amplification/buffer circuitry. Embodimentsof the present invention call for canceling the feedback current signaland using the cancelled signal as a feedback in an active impedanceloop. The active impedance loop is capable of reacting to an outputimpedance seen by an amplifier circuit and reacting to the impedance tocancel a substantial portion of the feedback signal using an activeimpedance feedback circuitry to feedback a smaller signal. Therefore, alower amount of noise may be experienced by a signal conditioningcircuit and improvements in power consumption may be realized. Utilizinga differential feedback configuration, longitudinal balance in a signalconditioning circuit may be achieved.

An impedance circuitry that comprises an impedance unit that isproportional to an output impedance seen by a signal conditioningcircuit, along with one or more sense resistors that are proportional tothe output impedance, may be used to perform an active impedancefeedback. For example, embodiments of the present invention call fordetermining a turns ratio N of an output impedance (e.g. a transformer)and implementing an active impedance feedback implementation that isproportional to N. An output voltage signal is converted into a currentsignal in a feedback loop. Utilizing the active impedance circuit, afeedback current signal is summed such that the feedback current signalis cancelled as it is being fed back into an input buffer or amplifier.Therefore, better noise-to-signal ratio may be achieved. Also, thedynamic range of the signal conditioning circuit may be improved.Embodiments of the present invention also call for converting the outputsignal into a current signal and summing the current signals such that asmall signal (i.e., a cancelled current signal) is sent back into theinput buffer/amplifier as a feedback signal. Lower bandwidth amplifiersmay be implemented due to the operation in the current domain.Therefore, the amplifier experiences a lower gain, thereby reducingpower consumption. The current feedback configuration may be used tobroadband the amplifiers in a signal conditioning circuit. Further,using the current feedback in a differential feedback configuration mayprovide the added benefit of promoting longitudinal balance. Althoughembodiments of the present invention are described in the context of aline card implementation, the teachings provided by the invention may beimplemented into a variety of signal conditioning circuits in a varietyof electronic/electrical applications.

Referring now to the drawings and in particular to FIG. 1, an apparatus100 in accordance with the present invention is illustrated. Theapparatus 100 includes a central office 110 that is coupled to asubscriber station 120 via a subscriber line 130. The central office 110and the subscriber station 120 are capable of sending and receiving asignal comprising a voice and data band. The voice band, as used herein,refers to a POTS voice signal ranging from 0–4 KHz. The data band refersto frequencies above the voice band, and may include, for example, thefrequency range employed in xDSL technologies. In one embodiment, thesubscriber line 130 may be a Public Switched Telephone Network (PSTN)line, a Private Branch Exchange (PBX) line, or any other medium capableof transmitting signals.

The subscriber station 120 may be a telephonic device capable ofsupporting pulse dialing. The term “telephonic device,” as utilizedherein, includes a telephone, or any other device capable of providing acommunication link between at least two users. In one embodiment, thesubscriber station 120 may be one of a variety of available conventionaltelephones, such as wired telephones and similar devices. In analternative embodiment, the subscriber station 120 may be any “device”capable of performing a substantially equivalent function of aconventional telephone, which may include, but is not limited to,transmitting and/or receiving voice and data signals. Examples of thesubscriber station 120 include a data processing system (DPS) utilizinga modem to perform telephony, a television phone, a wireless local loop,a DPS working in conjunction with a telephone, Internet Protocol (IP)telephony, and the like. IP telephony is a general term for thetechnologies that use the Internet Protocol's packet-switchedconnections to exchange voice, fax, and other forms of information thathave traditionally been carried over the dedicated circuit-switchedconnections of the public switched telephone network (PSTN). One exampleof IP telephony is an Internet Phone, a software program that runs on aDPS and simulates a conventional phone, allowing an end user to speakthrough a microphone and hear through DPS speakers. The calls travelover the Internet as packets of data on shared lines, avoiding the tollsof the PSTN.

Turning now to FIG. 2, a line card 210 and a DSL modem 220 areillustrated in accordance with the present invention. In one embodiment,the line card 210, which is integrated into the central office 110, iscoupled with the DSL modem 220, which resides within the subscriberstation 120. Because voice and/or data can be transmitted on thesubscriber line 130, the signal received and transmitted by the linecard 210 and the DSL modem 220 may include voice and data bandfrequencies.

The line card 210 may be located at a central office 110 or a remotelocation somewhere between the central office 110 and the subscriberstation 120 (see FIG. 1). The line card 210 services the subscriberstation 120, which in the illustrated embodiment is a telephonic device.The line card 210 is capable of processing DC voltage signals and ACsignals. The subscriber line 130 in the instant embodiment is atelephone line. The combination of the telephone device (subscriberstation 120) and the telephone line (subscriber line 130) is generallyreferred to as a subscriber loop.

The line card 210, which may be capable of supporting a plurality ofsubscriber lines 130, performs, among other things, two fundamentalfunctions: DC loop supervision and DC feed. The purpose of DC feed is tosupply enough power to operate the subscriber station 120 at thecustomer end. The purpose of DC loop supervision is to detect changes inDC load, such as on-hook events, off-hook events, rotary dialing, or anyother event that cause the DC load to change. In the interest of clarityand to avoid obscuring the invention, only that portion of the line card210 that is helpful to an understanding of the invention is illustrated.

Turning now to FIG. 3, one embodiment of the line card 210 isillustrated. In one embodiment, the line card 210 comprises a subscriberline interface circuit (SLIC) 310 as well as a subscriber lineaudio-processing circuit (SLAC) 320. The SLIC 310 performs a variety ofinterface functions between the line card 210 and the subscriber line130. The SLIC 310 is also capable of performing a variety of functions,such as battery feed, overload protection, polarity reversal, on-hooktransmission, and current limiting. The SLIC 310 is connected to theSLAC 320. The SLAC 320 is capable of processing analog-to-digital (A/D)and digital-to-analog (D/A) signal conversion, filtering, feed control,and supervision.

Turning now to FIG. 4, a more detailed description of the line card 210in accordance with one embodiment of the present invention isillustrated. In one embodiment, the SLIC 310 comprises a signalamplifier 410. The signal amplifier 410 is capable of amplifying anoutput signal sent by the line card 210. The signal amplifier 410provides for processing and amplifying the communication signal in sucha way that reduced power and lower noise levels are achieved. The signalamplifier 410 also receives communication signals, buffers them, andprovides an output buffering stage for driving the receivedcommunication signals. The signal amplifier 410 comprises a differentialactive feedback circuit 420 that is capable of providing an activeimpedance feedback described above in a differential feedbackconfiguration. A more detailed description of the circuitry relating tothe differential active feedback circuit 420 is provided in the drawingsand related description below.

Turning now to FIG. 5, a block diagram representation of one embodimentof the signal amplifier 410 is illustrated. The signal amplifier 410comprises a set of input buffers 510. The input buffers 510 receive adifferential input signal that comprises an input positive portion of aline 515 (input_pos) and an input negative portion on a line 517(input_neg). The input buffers 510 buffer the differential input signalsand process them using a signal conditioning unit 530, which provides adifferential output signal to a load impedance 550. The load impedance550 may comprise an equivalent impedance of a transmission line, whichmay be experienced through a transformer, affecting the output impedanceseen by the signal conditioning unit 530.

The signal conditioning unit 530 receives a differential feedback signalpair 560, which is fed-back differentially from the signal that is sentto the load. The differential feedback signal pair 560 is generallycurrent signals. The signal conditioning unit 530 comprises avoltage-to-current conversion unit 570, which converts the differentialvoltage pair 560 into a differential current signal. In one embodiment,the signal condition unit 530 sums components of the differentialcurrent signal for performing cancellation, thereby promotinglongitudinal balance. The signal conditioning unit 530 may also comprisean active feedback network 540. The active feedback network 540 iscapable of extracting the differential voltage signal from theconversion unit 570 and providing a differential active feedbacksynthesis such that a small differential signal pair 570 is fed-backinto an amplifier, which may be housed within the input buffers 510. Amore detailed description of the various circuits relating to thecomponents shown in FIG. 5 is provided in greater detail below.

Turning now to FIG. 6, a block diagram representation of theimplementation of the signal amplifier 410 described in the presentinvention is illustrated. The block diagram illustrated in FIG. 6generally illustrates the processing of communication signals receivedand transmitted by the line card 210 using the signal amplifier 410 inthe SLIC 310. In one embodiment, the block diagram of FIG. 6 may beimplemented in a semiconductor device (e.g., a semiconductor chip).

The downstream (DS) signal voltage (downstream relative to the line card210) is presented at the DDWN+/−pins (on a line 610 a and a line 610 b),where it is converted into a differential current by a datadown block620, which drives a set of output buffers 621. In one embodiment, theoutput buffers 621 have inverting inputs. Generally, the output buffers621 are configured as transconductance amplifiers, therefore, the DSsignal is re-converted into a differential voltage at output pins AY 630a and BY 630 b. In one embodiment, the gain applied to the DS signal mayprovide a signal that may be approximately 48 volts peak to peak fromDDWN+/−610 a, 610 b to AY 630 a and BY 630 b. The output voltage at AY,BY 630 a, 630 b drives a step-down transformer 635, which has a turnsratio of N:1, through a pair of current sensing resistors, Rs 632 a, 632b.

A datasense block 640 implements the communications signal path. Twotransconductance stages within the datasense block 640 convert thevoltage signal across AD, ABAL 638 a, 642 and BD, BBAL 638 b, 643 into adifferential current signal pair, DUP+645 a and DUP−645 b. External loadresistors 648 a, 648 b convert the signal back to a signal voltage. Asthe voltage signal across AD, ABAL 638 a, 642 and BD, BBAL 638 b, 643becomes proportional to the loop current (including the turns ratio, N),the transfer function from loop current to the differential currentsignal pair DUP+645 a and DUP−645 b becomes a current gain. The currentgain may be fixed for a given set of external components.

The circuit comprises a pair of sense resistors (NRs 650 a and NRs 650b) that are proportional to the turns ratio N, and an active impedanceNZ_(L) 750 to provide an approximate cancellation of the DS signalacross AD, ABAL 638 a, 642 and BD, BBAL 638 b, 643. The cancellation ofthe DS signal allows the datasense block 640 to operate without havingto process the full DS signal. This significantly reduces the dynamicrange requirements of the datasense block 640, which in one embodiment,may be designed to cope with a substantially worst-case cancellation of12 dB.

Additional current outputs from the datasense block 640 may drivefeedback signal currents into summing nodes of the output buffers 621blocks. This forms a feedback loop, which sets the terminating impedanceacross AD 638 a to BD 638 b. In one embodiment, provided that Rs 632 a,632 b is substantially made equal to N²*6.2 ohms, the impedance at AD638 a, BD 638 b may be N²*100 ohms. Since the feedback loop describedabove generally does not respond to DS signals, it does not control theterminating impedance for DS signals. Generally, the impedance may below in this case. However, less than ideal echo cancellation may resultin some variability of the DS voltage gain.

The circuitry shown in FIG. 6 comprises an active feedback network 540.The active feedback network 540 is capable of providing an activeadjustment to the impedance provided by the transformer 635 such that acancelled signal is fed back via the A balance and the B balanceterminals 642, 643. The node 646 (node G) is created by summing theoutputs from the datasense 640. The summing of the signals on node G646, which is fed back to the output buffers 621 provides a cancelledsignal feedback, which is a signal that is smaller than what it would bewithout cancellation. The components of the active feedback network 540is based upon the turn ratio N of the transformer 635. In other words,the value of the components of the active feedback network 540 isproportional to the turns ratio N. In one embodiment, the number N is alarge number, such as 100. Therefore, if a load is 100 times bigger thanthe components of the active feedback network 540, the operating poweris only affected by 1%. Hence, using the active feedback network 540,the feedback signal is converted into a current domain and then summedon the node G 646, such that the signal that is fed back to theamplifiers are essentially cancelled and very small. A more detailedimplementation of the active feedback network 540 is provided in FIG. 7and accompanying description below.

Turning now to FIG. 7, one embodiment of a circuit implementing theactive feedback network 540 in accordance with embodiments of thepresent invention is illustrated. A differential input voltage input_posand input_neg 515, 517 are provided to a buffer 710. The buffer 710 mayconvert the received input voltage signal into a current signal. Theoutput of the buffer on a line 715 is then fed to an amplifier 720 witha negative feedback configuration. An output terminal of the amplifier720 is coupled with a terminal of the sense resistor 632 a. The secondterminal of the sense resistor 632 a (node B) is then fed back to avoltage current amplifier located within the datasense 640, which maycomprise amplifiers 640 a and 640 b). Furthermore, the amplifier 640 aand 640 b may comprise one of more amplifiers and/or buffers built ineach one of them. The amplifier 640 a then provides an output feedback,which may be the positive terminal of a differential output feedbacksignal. The sense resistor 632 b provides a node D that is coupled to aninput of a voltage-to-current amplifier 640 a within the datasense 640.The outputs of the amplifiers 640 a, 640 a are then provided to theterminals 645 a, 645 b (shown, in FIG. 6). An output of the datasensesignal 640 a, which forms a node G (745), is summed with an output (nodeE) coming from the voltage-to-current signal amplifiers 640 a and 640 a.The summation of the signals on node G (745) is then fed back to theamplifier 720 as a current signal.

The active feedback network 540 provides a signal on a node A (755) anda node C (757) to the datasense voltage-to-current amplifiers 640 a, 640b, respectively. A terminal of the resistor R_(S) 632 a is coupled witha terminal of a first resistor 730 (N*R_(S)) that is proportional to thesense resistor 632 a by a factor of the turns ratio N of the transformer635 (shown in FIG. 6). Similarly, a second resistor 740 (N*R_(S)) isalso proportional to the resistor R_(S) 632 b by a factor of N.Additionally, the active feedback network 540 comprises an impedancedevice that provides an active impedance 750 (NZ′_(L)), which isproportional to the output impedance load Z_(L) 760. The activeimpedance 750 is proportional to the impedance load 760 by a factor ofN. The second terminal of the resistor 730 is coupled with an inputterminal of the active impedance 750, forming the node A 755. Similarly,the second terminal of the resistor 740 is coupled with the secondterminal of the impedance 750 forming node C 757. Node A 755 and node C757 are fed into the datasense voltage-to-current amplifiers 640 a, 640b. A first output of the datasense amplifiers 640 a and 640 b are summedon the node G 745 and fed back to the amplifier 720 via the node F 747,which is coupled to the line 715. In one embodiment, the voltage atnodes A and B, (755 and 717) are substantially the same. Additionally,the voltage at node C (757) and D (759) are also substantially the same.

The feedback that is sent to the amplifier 720 is a summation of thesignal at node A 755 and the negative version of the signal on node B717. In other words, the feedback signal, which is denoted by the signalI_(pos) on the node G 745, is the signal on node A 755 minus the signalon node B 717. The feedback signal (node A 755 minus node B 717) issubstantially small, i.e., very close to zero. Similarly a second outputfrom the voltage-to-current amplifiers 640 a, 640 b, may be summed on anode H (coupled to a line 749) and fed-back to an amplifier 790, whoseother input may be a reference voltage. The feedback signal, which isdenoted by the signal I_(neg) on the node H 749, is the signal on node C757 minus the signal on node D 759. The feedback signal (node C 757minus node D 759) is substantially small, i.e., very close to zero. Thisfeedback level is achieved if the circuit illustrated in FIG. 7 isproperly balanced.

One factor that leads to the proper balancing of the circuit in FIG. 7is a function on how accurate is the proportional impedance 750 alongwith resistors 730 and 740. When the active feedback network 540 is setat correct proportional levels, and balance of the circuit is achieved,the feedback signal is substantially zero, thereby providing for smallernoise effects and the other benefits described above. In other words,the feedback provided by the circuit illustrated in FIG. 7 provides fora cancelled signal as a feedback signal for improved operation of thecircuit.

By utilizing the small signals, the dynamic range of the amplifiers ofFIG. 7 may be reduced, therefore, lower power amplifiers are implementedreducing power, and less noise is experienced. One of the advantages ofmanipulating the voltage signals and utilizing them as current signalsby the circuit in FIG. 7 occurs because from a voltage point of view,the circuit “sees” a low gain and the impedance of the current sourceare very high. Therefore, the gain of the amplifier is near unity fromthe voltage feedback point of view. Hence, employing the electricalscheme described above, an amplifier will typically have a single poleroll-off. In order to keep the amplifier stable, since it is difficultto determine the actual gain, compensation for a unit gain isimplemented; wide bandwidth may be achieved by putting different amountsof compensation in the amplifiers of the circuit in FIG. 7. Utilizingthe current implementation, we are actually taking some of the currentfrom the output of the circuit and summing it into the feedback signal,which provides for better matching and balance on the two sides of theimpedance loop. Therefore, employing embodiments of the presentinvention, reduced dynamic range, power, and noise advantages areachieved, therefore, smaller circuits may be used, and savings in powerand space realized.

Turning now to FIG. 8, a flow chart illustration of embodiments of thepresent invention is illustrated. Upon receiving the signal, the signalamplifier 410 converts the output voltage into a current signal formanipulation of a current signal to reduce the dynamic range of theamplifiers being implemented. The signal amplifier 410 also conditionsthe received signal, which is in a differential feedback format, usingan active feedback network 540 (block 820). The feedback signal may thenbe converted to a differential current signal for summation andcancellation (block 830). The active feedback network 540 provides foran active feedback synthesis to provide a summation of the feedbacksignal such that it is reduced substantially, and is fed back to anamplifier in the circuit (block 840). The conditioned differentialfeedback signal is then fed-back into the amplifier providing for lessnoise amplification and more efficient processing of the receivedsignal. Utilizing embodiments of the present invention, a more robust,lower power, lower noise signal amplifier may be achieved. Therefore, amore efficient SLIC 310 may be used using embodiments described in thepresent invention. Therefore, lower line cards 210 may be realized.

Although for illustrative purposes, embodiments of the present inventionhave been discussed in the context of a line cards application, theamplifier arrangements taught by embodiments of the present invention isnot limited to line card applications. The concepts taught byembodiments of the present invention may be utilized in a variety ofelectronic applications. The apparatuses 110, 120, 130 may be integratedin a system capable of transmitting and receiving signals having a voiceband and/or a data band. The teachings of the present invention may beimplemented in a line card 210 that supports POTS technology, ADSLtechnology, and/or similar technologies. The teachings of the presentinvention may also be implemented in various other electronicsapplications.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1. A method, comprising: receiving a signal; and performing a currentfeedback of an output signal in an active feedback synthesis mode basedupon said signal, performing said active current feedback comprisingusing an active feedback network for conditioning an output signal,converting said signal into a differential current signal, and summingat least two components of said differential current signal forperforming a differential feedback.
 2. The method of claim 1, whereinreceiving said signal further comprises receiving a telecommunicationssignal.
 3. The method of claim 1, further comprising: providing a firstfeedback resistor pair that is proportional to a first output senseresistor; providing a second feedback resistor pair in series with saidfirst feedback resistor, said second feedback resistor beingproportional to a second output sense resistor; providing a feedbackimpedance in series with said first and second feedback resistor pairs,said feedback impedance being proportional to an output impedance; and asignal summing node pair to sum said components of said conditioneddifferential output signal.
 4. The method of claim 3, wherein performinga signal summation of at least two components of said conditioned outputsignal for feedback further comprises generating a canceled differentialfeedback signal by canceling a first signal from a first datasenseamplifier output that is based upon a first component of saidconditioned output signal with a second signal from a second datasenseamplifier output that is based upon a second component of saidconditioned differential output signal, and performing a differentialfeedback operation using said canceled feedback signal.
 5. An apparatus,comprising: means for receiving a signal; and means for performing ancurrent feedback of an output signal in an active feedback synthesismode based upon said signal, performing said active current feedbacksynthesis comprising using an active feedback network for conditioningan output signal, converting said signal into a differential currentsignal, and summing at least two components of said differential currentsignal for performing a differential feedback.
 6. An apparatus,comprising: a first amplifier and a second amplifier to buffer adifferential input signal to generate a differential output signal; andan active impedance network to condition said differential output signaland sum at a first component and a second component of a first portionof said conditioned differential output signal, and to sum first andsecond components of a second portion of said conditioned differentialoutput signal for generating a differential feedback signal for feedbackinto said first and second amplifiers.
 7. The apparatus of claim 6,further comprising: a first and a second sense resistor pairs fordetecting a current level of said differential input signal; a firstdatasense amplifier operatively coupled with a terminal of said firstsense resistor and a first terminal of said active impedance network,said first datasense amplifier to generate said first portion of saidconditioned output signal; and a second datasense amplifier operativelycoupled with a terminal of said second sense resistor and a secondterminal of said active impedance network, said second datasenseamplifier to generate said second portion of said conditioned outputsignal.
 8. The apparatus of claim 6, wherein said signal summation ofsaid first and said second components of said first and second portionsprovides a canceled differential feedback signal.
 9. The apparatus ofclaim 6, wherein said active impedance network comprises a firstresistor, a feedback impedance unit in series with said first resistor,and a second resistor in series with said feedback impedance unit, saidfirst resistor, second resistor, and said feedback impedance unit eachbeing proportional to an output impedance of said apparatus.
 10. Theapparatus of claim 9, wherein said output impedance is comprised of atransformer.
 11. The apparatus of claim 10, wherein said first resistor,second resistor, and said feedback impedance unit each beingproportional to a turns ratio of said transformer.
 12. The apparatus ofclaim 11, wherein first resistor, second resistor, and said feedbackimpedance unit each being proportional to said output impedance by afactor of
 100. 13. An apparatus, comprising: a voltage-to-current signalconverter to convert a differential input voltage signal; a first bufferamplifier operatively coupled to said voltage to current signalconverter, said first buffer amplifier to receive a component of aconverted differential input signal on a first input terminal andproduce a first buffered current signal; a first sense resistoroperatively coupled with said buffered amplifier; a second bufferamplifier operatively coupled to said voltage-to-current signalconverter, said second buffer amplifier to receive a component of saidconverted differential input signal on a second input terminal andproduce a second buffered current signal; a second sense resistoroperatively coupled with said second buffer amplifier; an activefeedback impedance network operatively coupled with said first andsecond sense resistors; a first datasense amplifier operatively coupledto said active feedback impedance network and to said first senseresistor, said first datasense amplifier to generate a first and secondcomponent of a differential feedback signal; a second datasenseamplifier operatively coupled to said active feedback impedance networkand to said second sense resistor, said second datasense amplifier togenerate a third and fourth component of said differential feedbacksignal; a first summing node for summing said first and secondcomponents of said differential feedback signal to generate a firstcomponent of a summed differential feedback signal for feedback intosaid first buffer amplifier; and a second summing node for summing saidthird and fourth components of said differential feedback signal togenerate a second component of said summed differential feedback signalfor feedback into said second buffer amplifier.
 14. The apparatus ofclaim 13, wherein said summed differential feedback signal is a canceleddifferential feedback signal.
 15. The apparatus of claim 13, whereinsaid active impedance network comprises a first resistor, a feedbackimpedance unit in series with said first resistor, and a second resistorin series with said feedback impedance unit, said first resistor, secondresistor, and said feedback impedance unit each being proportional to anoutput impedance of said apparatus.
 16. The apparatus of claim 15,wherein said output impedance is comprised of a transformer.
 17. Theapparatus of claim 16, wherein said first resistor, second resistor, andsaid feedback impedance unit each being proportional to a turns ratio ofsaid transformer.
 18. The apparatus of claim 17, wherein said turnsratio of said transformer is
 100. 19. A system, comprising: a subscriberline; and a line card electronically coupled with said subscriber line,said line card being adapted to: receiving a signal; and performing acurrent feedback of an output signal in an active feedback synthesismode based upon said signal, performing said active current feedbackcomprising using an active feedback network for conditioning an outputsignal, converting said signal into a differential current signal, andsumming at least two components of said differential current signal forperforming a differential feedback.
 20. The system of claim 19, whereinsaid line card comprising: a first amplifier and a second amplifier tobuffer a differential input signal to generate a differential outputsignal; and an active impedance network to condition said differentialoutput signal and sum at a first component and a second component of afirst portion of said conditioned differential output signal, and to sumfirst and second components of a second portion of said conditioneddifferential output signal for generating a differential feedback signalfor feedback into said first and second amplifiers.
 21. The system ofclaim 20, said line card further comprising: a first and a second senseresistor pairs for detecting a current level of said differential inputsignal; a first datasense amplifier operatively coupled with a terminalof said first sense resistor and a first terminal of said activeimpedance network, said first datasense amplifier to generate said firstportion of said conditioned output signal; and a second datasenseamplifier operatively coupled with a terminal of said second senseresistor and a second terminal of said active impedance network, saidsecond datasense amplifier to generate said second portion of saidconditioned output signal.
 22. The system of claim 21, wherein saidsignal summation of said first and said second components of said firstand second portions provides a canceled differential feedback signal.23. The system of claim 22, wherein said active impedance networkcomprises a first resistor, a feedback impedance unit in series withsaid first resistor, and a second resistor in series with said feedbackimpedance unit, said first resistor, second resistor, and said feedbackimpedance unit each being proportional to an output impedance of saidapparatus.
 24. The system of claim 23, wherein said output impedance iscomprised of a transformer.
 25. The system of claim 24, wherein saidfirst resistor, second resistor, and said feedback impedance unit eachbeing proportional to a turns ratio of said transformer.
 26. The systemof claim 25, wherein said first resistor, second resistor, and saidfeedback impedance unit each being proportional to said output impedanceby a factor of
 100. 27. A method, comprising: receiving a signal;converting said signal into a differential current signal; conditioningan output signal that is based upon said differential current signal,using a feedback network; and summing at least two components of saidoutput signal for performing a differential feedback.
 28. The method ofclaim 27, wherein summing said at least two components of saidconditioned output signal comprises generating a canceled differentialfeedback signal by canceling a first signal from a first datasenseamplifier output that is based upon a first component of saidconditioned output signal with a second signal from a second datasenseamplifier output that is based upon a second component of saidconditioned differential output signal, and performing a differentialfeedback operation using said canceled feedback signal.